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     CIRKIT MODEM 1200 TO AMSTRAD
       CPC464/664/128 INTERFACE

HARDWARE SPECIFICATION
_______________________________________

Introduction

This  document  presents  the  hardware
specification   for  a  minimal   parts
serial  interface to allow  the  Cirkit
Modem  1200  to  communicate with   the
Amstrad range of CPC computers.
_______________________________________

Features

o  Asychronous operation
o  5, 6, 7 or 8 bit characters
o  1, 1.5 or 2 stop bits
o  Odd or even or no Parity, generation
   and checking
o  Framing,  overrun and  parity  error
   detection
o  Carrier Detect input (/DSR)
o  MUTE  output (Intended for  Half  /
   Full    duplex switching)
o  1200/1200,  1200/75,  300/300   bps
   operation
o  RS232   compatible   inputs,    TTL
   compatible outputs
_______________________________________

The Hardware

An  8251 UART is used as the basis  for
the  design, associated with  this  are
four support ICs.
_______________________________________

CPU Interface

The   4MHz   clock  output   from   the
expansion bus is used to derive the CLK
input for the 8251.

The  active  high 8251 RESET  input  is
derived from the bus /RESET line.

Two  OR  gates  provide  the  necessary
address decoding.
_______________________________________

I/O Address Map

Two  addresses  are used from  the  I/O
address  map.  The addresses  used  are
those   recommended   by   Amsoft   for
communications  peripherals,  and   are
therefore  compatible with all  Amstrad
peripherals,  with  the  exception   of
other communications devices.

ADDRESS   READ        WRITE
==================================
0FADFH    DATA IN     DATA OUT
0FBDFH    STATUS      CONTROL


0FADFH    Data input/output register

0FBDFH    Status / Control register
When read, returns the UART status.

/DSR is used as a Carrier Detect input.

When  written  after  a  soft  or  hard
reset,  accesses the  Mode  Instruction
register  are used to define  operating
parameters.

After  writing  the  Mode   Instruction
word,  this address then  accesses  the
Command instruction register.

/DTR  is used in conjunction  with  the
selected clock ratio to set the desired
baud rate. /RTS is used as a MUTE line,
to  switch the Modem into half or  full
duplex.  D7 should be sent as a 0.

Refer  to a data sheet on the 8251  for
detailed   description   of    register
assignments.
_______________________________________

Modem Interface

Four signals control data transfer  via
the Modem and 8251, these are:

Transmit Data
TTL  level  Output.   0=Mark,   1=Space
(+5V). Data to the Modem.

Receive Data
TTL level Input.  +5V=Space, 0V=Mark
Data from the Modem.  Diode clamped  to
allow RS232 compatability.

Mute
TTL level Output.  +5V=Mute
This  signal is driven directly by  the
8251   /RTS   output.    The    Modem's
transmitter  should  be  disabled  when
MUTE is TTL logic 1.

Carrier detect
TTL level Input.  0V=Carrier Detected
Carrier detect input from modem.  Diode
clamped for RS232 compatibility. Should
be connected to 0V if not in use.
_______________________________________

Cable assignment

The  modem interface lines are  brought
out along a 6 core cable to a 5 pin DIN
plug  to allow a Modem or other  device
to  be connected.  Pin  connections  as
follows:

Black  -  Ground 0V
Brown  -  Transmit data (to Modem)
Red    -  Receive  data (from Modem)
White  -  +5V from computer
Yellow -  MUTE (to Modem)
Green  -  Carrier Detect (from Modem)
Shield -  Ground 0V
_______________________________________

5 Pin DIN Connections

The  pin connections to the 5  pin  DIN
are:

1    -    Ground 0V
2    -    Transmit data (to Modem)
3    -    Receive  data (from Modem)
4    -    Not connected
5    -    MUTE (to Modem)

Carrier  Detect  connected  to   ground
(Green).
_______________________________________

Baud Rate Generation

Three   fundamental  baud   rates   are
supported,  these  are derived  from  a
614KHz oscillator.  Two frequencies  of
1200Hz and 19.2KHz are used to  support
(RX/TX) 1200/1200, 300/300 and  1200/75
baud operation.

The   8251  receive  clock  is   always
19.2KHz.   The  transmit clock  can  be
selected from any of the two, by use of
the 8251 /DTR output.  The table  below
details how the baud rates are derived.

BAUD RATE
RX    TX        /DTR    CLOCK RATIO
1200  1200        0          16
300   300         0          64
1200  75          1          16

All  three available baud rates may  be
used  in  either half  or  full  duplex
mode.   It  is the  responsiblility  of
support  software to control  the  MUTE
output, for half duplex operation.
_______________________________________

